In conventional systems for blow-molding articles such as hollow plastic containers, an extrusion die head contains orifice tooling that is controlled by a parison programmer to vary the wall thickness according to a predetermined thickness profile lengthwise of the parison. The hollow extruded parisons are fed in sequence to blow molding apparatus, typically comprising a plurality of mold-half pairs that pass in sequence adjacent to the extrusion head for receiving the parisons. A blow molding system of this character is disclosed, for example, in U.S. Pat. No. 4,523,904. The purpose of the parison programmer is to control the orifice tooling to provide a parison wall thickness that changes along the length of the parison in correspondence with changes in material requirements along the length of the molded container. Since any change in wall thickness also changes the length of the parison in a free-extrusion system, or the stretch of the parison in a captured-extrusion system, it is difficult to coordinate programmer position along the parison relative to the desired position in the molded container (i.e., programmer "phase" synchronizations).
It has heretofore been proposed to attempt to synchronize operation of the parison programmer by making a radical profile change at a specific point along the profile, and then checking the position of this radical change in the molded container. However, the act of making such a radical change itself tends to change synchronization or phase of the parison profile. Furthermore, all containers made during this process are not usable and must be scrapped. It is a general object of the present invention to provide a method and apparatus for synchronizing operation of the parison programmer in a blow-molding system of the described character that may be readily implemented in existing systems without substantial modification or expense, and that may be readily employed by relatively unskilled personnel. Another and more specific object of the present invention is to provide a method and apparatus of the described character that automatically synchronizes operation of the parison programmer to the molding apparatus in a closed loop system without operator intervention.
In a blow-molding system in which a parison programmer is coupled to orifice tooling in an extrusion die head for controlling wall thickness of an extruded parison fed to a blow-molding apparatus, a method of synchronizing operation of the parison programmer in accordance with the present invention contemplates placement of observable indicia on the parison after the parison exits the die head. This indicia placement preferably is carried out under control of the parison programmer to place the indicia at a predetermined position along the parison profile, such that mis-positioning of the indicia indicates absence of phase synchronization of the parison programmer. The indicia is then observed on a molded article to determine phasing of the parison programmer relative to the molding apparatus, and operation of the parison programmer is adjusted to correct any mis-positioning. In the preferred implementation of the invention, the readable indicia comprises an optically readable spot or dot, and position of this spot or dot on the molded article is determined by an optical sensor that provides an electrical signal for automatic closed-loop correction of parison programmer phasing without operator intervention.
Apparatus for synchronizing operation of the parison programmer in a system for blow-molding hollow plastic articles such as containers includes an ink jet device coupled to the parison programmer for placing optically readable indicia on parisons as they exit the extrusion die head at a preselected position lengthwise of the parisons coordinated with wall thickness profile programming prestored in the parison programmer. An optical sensor observes position of this optically readable indicia on containers molded by the molding apparatus, and operation of the parison programmer and orifice tooling relative to the die head and the molding apparatus is altered, preferably automatically, so as to place the indicia at a predetermined location relative to the molded container. In the preferred embodiment of the invention, the optically readable indicia is placed on a portion of the molded container that will be subsequently removed and discarded (or recycled), together with a scale molded into the container to facilitate operator observation of parison programmer phasing .